In encoders for measuring rotation, patterns or scales which permit absolute or incremental motion sensing are typically applied to discs which are physically rotated about a central axis to permit electro-optical sensing of motion or position. It will be appreciated that the location of the pattern relative to the central axis requires that a centrally positioned hole or aperture be precisely located with respect to the pattern so that the radial position of the pattern is accurately established regardless of the angular position of the disc on a spindle. Moreover, for precise positioning, the interior side wall or walls of the aperture in the disc must be exceptionally uniform and straight so that when the disc is positioned over a precisely manufactured spindle, the pattern may be accurately referenced to the central rotational axis of the spindle.
In the past, encoder discs or bars have been manufactured, for instance, in accordance with the teachings of U.S. Pat. No. 3,905,817 issued Sept. 16, 1975 to Joseph J. Bakewell. As will be appreciated, in this patent the encoder disc or bars are made from glass and is patterned from a master disc. The finished disc has a plated metal pattern which provides opaque areas, with the rest of the glass encoder disc being light transmissive. Another type of mask is shown in U.S. Pat. No. 3,449,221 issued on June 10, 1969 to Lowell E. Thomas. Additionally, as illustrated in co-pending application Ser. No. 175,444 filed Aug. 4, 1980, by Luke R. Volpe, and assigned to the assignee hereof, an encoder disc or bar is provided with precisely placed reflective and non-reflective segments by providing the non-reflective segment in terms of a roughened or matte surface. This roughened surface is formed in one embodiment by selectively etching a master, with the matte or roughened surface being transferred to an overplated metallic layer when the layer is peeled off, the peelable layer comprising the completed scale. Peelable scales formed in this manner are exceptionally useful because of the ease with which they are manufactured and because of the accuracy with which the pattern on a master platten is transferred to the peelable layer.
When these peelable layers are provided for rotary encoding, it is essential that the patterned layer have a precisely located central aperture. It is also of importance to form the central aperture with a straight side or sides. It should be noted that boring or punching of the center hole can provide non-uniform aperture side walls which can preclude precise positioning of the patterned encoder disc.